An Introduction to Functional Programming Through Lambda Calculus

- 168 -In order to access the individual characters making up a string it must be unpacked into a list of single characterstrings. The standard function:does this:explode- explode;> fn : string -> (string list)For example:- explode "hello";> ["h","e","l","l",’o"] : string listSimilarly, the standard function:implodeconverts a list of strings to a single string:- implode;> fn : (string list) -> stringFor example:- implode ["Time ","for ","tea?"];> "Time for tea?" : stringNote that implode will join strings of any length.9.12. Comparison operatorsSML provides a variety of overloaded infix comparison operators. Equality and inequality are tested with:= and may be used with booleans, integers, strings, lists and tuples.The less than, less than or equal, greater than and greater than or equal operators:< = >may be used with numbers and strings. For strings, they test for alphabetic order, for example:- "haggis" < "oatcake";> true : boolSML systems may not display these operators’ types because they are overloaded.For all these operators, both arguments must be of the same type.9.13. Functions